In manufacturing of a laminated plate such as a printed circuit board, a method of inserting a laminated plate material 12 to be pressed between hot platens 13, 13 as heating/pressing means and applying a certain pressure and certain heat to the laminated plate material 12 as shown in FIG. 10 is used in a press forming process and a thermo-compression bonding process. In order to obtain a high precision formed product, it is necessary to uniformly apply heat and a pressure to the whole surface of the laminated plate material 12 in a hot press process. For this purpose, a hot press process is performed with a flat plate-like cushioning pad 11 interposed between each hot platen 13 and the laminated plate material 12.
General characteristics required for the cushioning pad 11 include: a cushioning property for absorbing concaves and convexes of the hot platens 13 and the laminated plate material 12; in-plane uniformity for uniformly transmitting a temperature and a pressure from the hot platens 13 to the laminated plate material 12 in the whole pressing surface; a heat transfer property for efficiently transferring heat from the hot platens 13 to the laminated plate material 12; and heat resistance to a press temperature.
Conventionally, a paper cushioning pad made by laminating about 3 to 20 sheets of kraft paper or linter paper has been commonly used as the hot press cushioning pad 11. The paper cushioning pad is inexpensive and has balanced physical properties in terms of the cushioning property, the in-plane uniformity, and the heat transfer property. The reasons for this are considered as follows: the paper contains an appropriate amount of voids, and this contributes to the cushioning property; constituent fibers of the paper are oriented substantially in the plane direction, and this contributes to the in-plane uniformity; and the paper has a thickness as small as about 0.1 to 1 mm per sheet, and this small thickness contributes to the heat transfer property. However, the paper cushioning pad does not have an ability to restore the voids after pressing, and the constituent fibers are degraded by heat. The paper cushioning pad is therefore disadvantageous in that it cannot be repeatedly used for a plurality of presses.
In view of this, various kinds of hot press cushioning pads such as organic or inorganic fibers bound by a binder, a rubber, a nonwoven fabric, a woven fabric, and a lamination of these materials have been proposed as hot press cushioning pads capable of repeated use.
Japanese Unexamined Patent Publication No. H04-361012 discloses a hot press cushioning pad produced by alternately laminating webs and base fabrics, needling the lamination, impregnating the resultant needle-punched nonwoven fabric with a heat-resistant resin, and heating and pressing the resin-impregnated needle-punched nonwoven fabric to a density of 0.6 g/cm3 to 0.9 g/cm3. A meta-aromatic polyamide fiber, hemp (ramie), and a polyester are used as a material of the nonwoven fabric, and a silicone rubber, an ethylene-acrylic rubber, and an EPDM are used as the heat-resistant resin. According to the structure of Japanese Unexamined Patent Publication No. H04-361012, a cushioning property which is stable over time even after repeated use in a plurality of presses can be obtained by defining the density to a specific range.
In such a needle-punched nonwoven fabric as used in Japanese Unexamined Patent Publication No. H04-361012, however, fibers are mechanically tangled by needling the webs. Therefore, the density of the webs themselves is not uniform. The hot press cushioning pad using the needle-punched nonwoven fabric therefore has a disadvantage of poor in-plane uniformity for uniformly transmitting a temperature and a pressure. Moreover, in the needle-punched nonwoven fabric, it is necessary to tangle the fibers by needling the webs in the thickness direction. Accordingly, the thickness of the hot press cushioning pad cannot be reduced very much. For example, the hot press cushioning pad described in the above publication has a final thickness of about 3.3 mm to about 3.7 mm, and therefore, has a disadvantage of a poor heat transfer property.
Moreover, in the hot press cushioning pad described in the above publication, the needle-punched nonwoven fabric is heated and pressed after impregnated with a heat-resistant resin. Therefore, the void ratio becomes small and the cushioning property is not so good. In other words, the hot press cushioning pad described in the above publication sacrifices the in-plane uniformity, the heat transfer property, and the cushioning property in order to make the physical properties stable over time even after repeated use.
Japanese Unexamined Patent Publication No. H04-197299 discloses a press cushioning pad formed by 3 to 5-ply multilayered woven fabrics using a heat-resistant spun yarn, and a press cushioning pad formed by impregnating the multilayered woven fabrics with a heat-resistant resin solution. In general, woven fabrics have an advantage of an accurate area weight, while having a disadvantage of a poor cushioning property. The cushioning pad described in Japanese Unexamined Patent Publication No. H04-197299, however, has a multilayered woven fabric structure to obtain a cushioning property. However, woven fabrics essentially have a poorer cushioning property than that of cotton-like nonwoven fabrics, and no significant improvement in cushioning property can be expected even if the multilayered woven fabric structure is provided. Moreover, if organic fibers are used as a material of the woven fabrics, dimensions change after repeated use, and sufficient stability is not obtained. When inorganic fibers such as glass fibers are used, on the other hand, excellent shape stability is obtained, but the fibers are bent or broken by a press, whereby the cushioning pad cannot be used repeatedly.